“Our next task is to kind of go up the pipe and figure out why is that happening?” McLellan asked. “Is it an illicit connection? Is it failing laterals in the area? Is it that the main sewer lines are leaking and failing?”

McLellan says she hopes to eventually tell communities where to best spend their sewer repair dollars. She says the bacteria that travels in sewage is the biggest immediate health risk, but longer-term, she also worries about chemicals and pharmaceuticals in the lake.

As people get ready to hit the beach in the next month or two, McLellan offers her advice: “My general rule of thumb is, if it's rained in the last 24 hours, I'm a little careful about swimming.”